During the last 20 years, various manufacturers have introduced a number of web formers operating by the twin-wire principles. A review of such web formers was published in the journal Pulp & Paper, Sep. 1982. In addition to the web formers mentioned in the afore-mentioned review, or in relation to them, reference is made to the following patent/publications: Canadian 960,496; Fed. Rep. Germany 2,105,613; U.S. Pat. No. 3,438,854; U.S. Pat. No. 3,846,232; U.S. Pat. 3,941,651; U.S. Pat. No. 3,997,390; U.S. Pat. No. 4,113,556; U.S. Pat. No. 4,154,645; and U.S. Pat. No. 4,609,435. Reference is also made to applicant's U.S. Pat. No. 3,996,098, corresponding to applicant's FI Patent Application No. 751774, as well as to applicant's FI Patent Application Nos. 843081 and 852291.
Applicant's FI Patent Application No. 751774 (corresponding to U.S. Pat. No. 3,996,098) and FI Patent Application Nos. 851650, 852662 and 902283, describe web-formers which are marketed by the applicant under the trademark "Speed-Former". The "Speed-Former" web formers can be characterized as roll-shoe formers, because the upper forming roll acts as a dewatering member to quite a limited extent only, because the twin-wire forming zone defined on it is quite narrow, and there are no means for drawing of the water around it. Thus, the main function of said upper roll is to make sure that the web is separated from the covering wire and follows the carrying wire.
In these web forming sections, the need for further development has occurred in particular in the case of papers manufactured out of slowly filtered pulp types, such as SC paper, and at particularly high machine speeds. In order to satisfy said needs of further development so that it is possible to guarantee a sufficiently high dry solids content after the wire part with all the pulp qualities used and with all web speeds, the web former described in the applicant's afore-mentioned FI Patent Application No. 902283 has been developed, in whose roll-shoe roll former a considerable proportion of dewatering is carried out on the last (second) forming roll, in whose connection the necessary dewatering equipment is fitted so that, on the last forming roll, the dry solids content of the web is increased further by about 3-5 percent.
With respect to the prior art related to the present invention, reference is further made to applicant's FI Patent Application No. 885609, and to the FI patent Application Nos. 885606 and 885607 of Valmet-Ahlstrom, Inc., in which web formers marketed under the trademark "MB-former" are described.
With increasing running speeds of paper machines, several problems in the web formation have been manifested with more emphasis. In the web former of a paper machine, the phenomena that affect the fiber mesh and the water, which is still relatively free in connection with the fiber mesh, such as centrifugal forces, are, as a rule, increased in proportion to the second power of the web velocity. The highest web speeds of the present day newsprint machines are of an order of 1200 m/min. However, newsprint machines are being planned in which a web speed of up to about 1500 m/min is aimed at.
It is therefore an object of the present invention to further development the web formers described in applicant's FI Patent Application No. 751774 (corresponding to U.S. Pat. No. 3,996,098) and FI Patent Application Nos. 851650, 852662 and 902283.
A further object of the invention is to provide a twin-wire gap former whose dewatering capacity and efficiency can be increased in comparison to the roll-shoe formers of the "Speed Former" type and with other, corresponding formers.
A further object of the present invention is to provide a twin-wire former in which an increased proportion of dewatering can be carried out on the first forming roll without deterioration of the formation.
It is a further object of the invention to provide a twin-wire former which is suitable for different paper qualities also be relatively thick paper qualities and for pulps whose dewatering is relatively difficult.
Further objects of the invention are to provide a former in which the formation of the paper produced is good and the porosity of the paper is low, i.e. there are no so-called pinholes in the paper.